[Tex/LaTex] Index under sum symbol below following equation

math-modemath-operators

I have a sum symbol with a quite long index underneath. This produces too much space between the following signs.

\begin{equation}
 \sum_{X\leq-VaR_\alpha(X)}{P(X)\cdot x} 
\end{equation}

I would like to see the index to reach under the P(X)x but in a very specific way: The X in the index should be left-aligned with the \sum symbol and any overlap of the index on the right of the symbol should displace the operand.

Is there any way to do this? I don't want to use a two lines index.

Best Answer

Another solution, also provided by the mathtools package, is to use its \smashoperator macro. In fact, by writing \smashoperator[r]{...}, one can arrange that the material overlaps only on the right, rather than on both sides. (Observe that the word "Note" does not overlap with the material below the summation sign.)

Note that I would also create a macro, called (say) \VaR, to typeset the string VaR with upright letters. And, do note that there's no point in encasing P(X)\cdot x in curly braces.

\documentclass{article}
\usepackage{mathtools}
\newcommand\VaR{\mathrm{VaR}}
\begin{document}
\begin{equation}
\text{Note:}\smashoperator[r]{\sum_{X\leq\VaR_\alpha(X)}} P(X)\cdot x  
\end{equation}
\end{document}

Addendum to address the OP's follow-up question: You may achieve your typesetting objective by (a) switching to the basic, i.e., centered version of the \smashoperator directive and inserting a suitable "kern" at the start of the subscript material. I know it's not fruitful to argue about tastes, but in my opinion the result of right-shifting the subscript material is anything but good-looking.

\documentclass{article}
\usepackage{mathtools}
\newcommand\VaR{\mathrm{VaR}}
\begin{document}
\[
\text{Note:}\smashoperator{\sum_{\mkern70mu X\leq\VaR_\alpha(X)}} P(X)\cdot x  
\]
\end{document}